The present invention relates to a magneto-optical head device for reproducing magneto-optically recorded data, and more particularly to reproduction of high-density recorded data.
A magneto-optical disk device is capable of storing a large capacity of data, and there has been demand for multi-media systems having further increased capacity. The inventors of the present application proposed an optical head device capable of achieving super resolution reproduction of data recorded on an optical disk by providing a light shading plate in an optical head unit of the magneto-optical disk device (U.S. Pat. No. 2,664,327).
In this optical head device, after a light beam emitted by a semiconductor laser as a light source is made parallel rays by a collimator lens and caused to have a circular cross section by a prism for correcting the axial symmetry of the laser, the resulting light beam transmits through a first beam splitter and is then condensed on a magneto-optical disk by an objective lens. On the magneto-optical disk, data is recorded by orienting the magnetization of a recording film in a direction corresponding to data to be recorded. By applying an external magnetic field to the recording film which has been heated to a temperature of not lower than its Curie temperature by the irradiation of the light beam, the magnetization of the recording film is oriented in a desired direction.
The reflected light from the magneto-optical disk is made parallel rays by the objective lens, and then incides again on the first beam splitter and is reflected. The reflected light is split into two beams by a second beam splitter so that one is used for a detection of a servo signal and the other is used for a detection of a reproduced signal. The light used for the detection of a reproduced signal transmits through a light shading plate and then incides on a reproduced signal detection unit. The reproduced signal detection unit includes a half-wave plate, a polarization beam splitter and photodiodes. The light shading plate is formed by providing an opaque film at substantially the center of a transparent substrate, so that light does not transmit through a portion where the opaque film is formed but transmits through the surrounding portion.
The light transmitted through the surrounding portion of the light shading plate is transmitted through the half-wave plate so that it is rotated at 45 degrees, and then split into a p-polarized component and an s-polarized component by the polarization beam splitter. When the light emitted by the light source is p-polarized light, the reflected light from the magneto-optical disk contains an s-polarized component as a signal component due to the magnetic Kerr effect. The p-polarized component and s-polarized component obtained by the transmission of the half-wave plate for the rotation of 45 degrees and the polarization beam splitter are received by the photodiodes, respectively, and the difference between the components is calculated to provide a reproduced signal.
In the magneto-optical head device with the above structure, when the reflected light from the magneto-optical disk transmits through the light shading plate, the central portion of the reflected light in the irradiated spot on the magneto-optical disk is shaded, thereby obtaining a super resolution effect on the reproduced signal. FIG. 1 shows an example of the amplitude characteristic of the transfer function of the magneto-optical head device. The vertical axis represents the degree of modulation, and the horizontal axis represents the spatial frequency. In the graph, a graph with the light shading plate shows the characteristics of the above-described magneto-optical head device, while a graph without the light shading plate shows the characteristics obtained by performing differential detection with the use of the whole reflected light for the detection of a reproduced signal. It will be understood from the graph that the magneto-optical head having the light shading plate at a spatial frequency ranging from 800 to 1000 (cycle/mm) achieves a higher degree of modulation. Therefore, high-density recorded data can be reproduced with excellent resolution by providing the light shading plate which shades the transmission of the central portion of light.
Hence, the magneto-optical head with the above-described structure can achieve super resolution reproduction of high-density recorded data. However, since a portion of light transmitted through the light shading plate is shaded, the amount of light received by the photodiodes is reduced. FIG. 2 is a graph showing the transmittance distribution of the light shading plate with the vertical axis representing the transmittance and the horizontal axis representing the distance from the center of the light shading plate. It will be understood from the graph that the transmittance is 0% in the area of radius r0 at the center of the light shading plate. Therefore, in comparison with the magneto-optical head without a light shading plate, the amount of light received by the photodiodes is lower, and the reproduced signal itself is lessened though the resolution of the reproduced signal is high. For this reason, the above-described magneto-optical head device with the light shading plate has a problem that the S/N of the reproduced signal deteriorates.
In view of the above circumstances, the present invention was made, and its object is to provide a magneto-optical head device capable of improving the S/N by magnifying a reproduced signal itself and achieving super resolution reproduction by the inclusion of light shading means for transmitting a predetermined polarized component at a higher ratio than other polarized component, or light shading means for transmitting the predetermined polarized component while shading the other polarized component.
A magneto-optical head device according to the present invention is a magneto-optical head device for obtaining a reproduced signal with the use of reflected light from a magneto-optical recording medium, and characterized by including light shading means on which the reflected light incides and providing the light shading means with a polarization film having a higher transmittance for a predetermined polarized component than for other polarized component in substantially a central portion of the incident reflected light.
Therefore, in substantially the central portion of the reflected light incident on the light shading means having the light shading film, i.e., the central portion of the optical amplitude distribution, a greater amount of the predetermined polarized component than the other polarized component is transmitted. Here, the predetermined polarized component refers to a polarized component which is not a signal component. Since the outputs of photodiodes during the detection of a reproduced signal are not proportional to the amplitude of light but are proportional to the power of light, the reproduced signal itself is magnified with an increase in the amount of light transmitted through the light shading means, and the S/N is improved. Moreover, since the transmittance of a polarized component as a signal component is low, super resolution reproduction can be achieved The magneto-optical head device according to the present invention is a magneto-optical head device including: an objective lens on which reflected light from a magneto-optical recording medium incides; a beam splitter for splitting the reflected light into a light beam for a reproduced signal detection unit and a light beam for a focusing error and tracking error detection unit; light shading means on which the reflected light incides; and the reproduced signal detection unit for detecting a reproduced signal with the use the reflected light transmitted through the light shading means, and characterized in that the light shading means is provided with a polarization film having a higher transmittance for a predetermined polarized component than for other polarized component in substantially a central portion of the incident reflected light, and disposed at such a position that the reflected light split by the beam splitter incides thereon.
Hence, after the reflected light is split by the beam splitter, one of the split light incides on the light shading means to transmit a predetermined polarized component, and the other is used in the focusing error and tracking error detection system. Since the outputs of the photodiodes during the detection of a reproduced signal are not proportional to the amplitude of light but are proportional to the power of light, the reproduced signal itself is magnified with an increase in the amount of light transmitted through the light shading means, and the S/N is improved. Moreover, since the polarized component as a signal component is shaded, super resolution reproduction can be achieved. Furthermore, in comparison with a structure in which the reflected light is split by the beam splitter after being shaded by the light shading means, the amount of light for use in the focusing error and tracking error detection is increased.
The magneto-optical head device according to the present invention is a magneto-optical head device including: an objective lens on which reflected light from a magneto-optical recording medium incides; a beam splitter for splitting the reflected light into a light beam for a reproduced signal detection unit and a light beam for a focusing error and tracking error detection unit; light shading means on which the reflected light incides; and the reproduced signal detection unit for detecting a reproduced signal with the use of the reflected light transmitted through the light shading means, and characterized in that the light shading means is provided with a polarization film having a higher transmittance for a predetermined polarized component than for other polarized component in substantially a central portion of the incident reflected light, and disposed at such a position that the reflected light transmitted through the light shading means incides on the beam splitter.
Hence, after transmitting the predetermined polarized component in the central portion of the optical amplitude distribution of the reflected light through the light shading means, the reflected light is split by the beam splitter so that one of the split light incides on the reproduced signal detection system and the other is used in the focusing error and tracking error detection system. Since the outputs of the photodiodes during the detection of a reproduced signal are not proportional to the amplitude of light but are proportional to the power of light, the reproduced signal itself is magnified with an increase in the amount of light transmitted through the light shading means, and the S/N is improved. Moreover, since the polarized component which is a signal component is shaded, super resolution reproduction can be achieved.
The magneto-optical head device according to the present invention is characterized in that the above-described polarization film is a dielectric multilayer film which transmits the predetermined polarized component but reflects or absorbs the other polarized component.
Thus, since the dielectric multilayer film is used as the polarization film, substantially 100% of the predetermined polarized component is transmitted, while substantially 100% of the other polarized component is reflected or absorbed. Here, the predetermined polarized component refers to a polarized component which is not a signal component, and the reproduced signal itself is further magnified as the amount of light transmitted through the light shading means is further increased, thereby improving the S/N. In addition, since the polarized component which is a signal component is shaded at a higher ratio, super resolution reproduction with higher resolution can be achieved.
The magneto-optical head device according to the present invention is characterized in that the predetermined polarized component selectively transmitted through the polarization film is polarized in the same direction as the polarization direction of the light beam irradiated on the magneto-optical recording medium.
Accordingly, the emitted light beam is irradiated on the magneto-optical recording medium, reflected by the magneto-optical recording medium, and then incides on the light shading means. When the polarization direction of this light beam is, for example, p polarization, an s-polarized component as a signal component is produced by the magnetic Kerr effect during the reflection of the light beam, and the p-polarized component is a polarized component which is not a signal component. Therefore, since the polarized component that is not a signal component is transmitted, the amount of light transmitted through the light shading means is increased, and the reproduced signal itself is magnified. Furthermore, since the polarized component which is a signal component is shaded, super resolution reproduction can be achieved.
The magneto-optical head device according to the present invention is characterized by including condensing means for condensing the reflected light transmitted through the light shading means.
Hence, since the polarized components contained in the reflected light are respectively transmitted through the light shading means with different transmittances between substantially the central portion and the surrounding portion of the light and then condensed, the central portion of the light and the surrounding portion of the light are optically superimposed and supplied to the reproduced signal detection unit. For this reason, since the central portion of the transmitted light is directly related to component a of light proportional to the magnitude of the reproduced signal, the component a is increased, and consequently the S/N of the reproduced signal is further increased. As the condensing means, for example, a condenser lens is used, and positioned between the light shading means and the reproduced signal detection unit in the optical path of the reflected light. Alternatively, the condenser lens may be positioned on the incident side of the light shading means. In this case, the distance between the condenser lens and the light shading means is preferably set within a focal distance of the condenser lens so that the reflected light is transmitted through the light shading means before it is focused.